Given an array of integers `heights` representing the histogram's bar height where the width of each bar is `1`, return the area of the **largest rectangle** in the histogram.
**Example 1:**
```
Input: heights = [2,1,5,6,2,3]
Output: 10
Explanation: The largest rectangle has area = 10 units (bars at index 2 and 3, heights 5 and 6).
```
**Example 2:**
```
Input: heights = [2,4]
Output: 4
```